1. Field of the Invention
The present invention generally relates to medical electronics devices, and more specifically relates to a lightemitting diode drive for use with an oximeter system.
2. Description of Prior Art
Oximeters have long been used to measure blood oxygen saturation. The greater the oxygen saturation, the redder are the red blood cells. If red light at a wavelength of 660 millimicrons (mu) is directed toward the blood sample, the amount of light reflected depends on how red the blood is, i.e., its oxygen content. The amount of reflected light increases with the oxygen saturation.
However, it has been recognized that an absolute measurement of the reflected red light is not a true indication of oxygen saturation. This is due to the fact that the oxygen content of the blood sample under test is affected not only by oxygen saturation, but also by the concentration of the hemoglobin in the blood. For example, even if the red blood cells are 100 percent saturated with oxygen, the amount of red light reflected may be very small if the concentration of red blood cells in the blood is low. For this reason, for many years the measurement of oxygen concentration has been performed by directing light at two wavelengths toward the blood sample. In addition to the 660-mu light, light at a wavelength of 805 millimicrons is also directed toward the sample (either simultaneously or sequentially with the 660-mu light). The amount of light at the higher wavelength reflected from the sample is dependent upon the concentration of the red blood cells in the sample, but is not affected by the oxygen saturation of these cells. Consequently, the 805-mu reflected light can be used as a reference to prevent the hemoglobin concentration from affecting the oxygen saturation measurement. Instead of reading only the amount of 660-mu light reflected from the sample, the ratio of the 805-mu reflected light to the 660-mu reflected light is measured. The red blood cell concentration affects both the numerator and the denominator of the ratio in the same way and thus does not affect the ratio itself. Thus the measurement is in effect made independent of the concentration of the red blood cells in the blood. Since only the denominator of the ratio is affected by the oxygen concentration, the ratio is an indication of the oxygen concentration.
Recently, light-emitting diodes (LEDs) have been employed as light sources for oximeters. In any oximeter utilizing two or more LEDs as two different light sources, (for example, as described above), it is important to maintain constant or proportional the intensities of the light emitted from the diodes. If the intensities of the two light emissions vary proportionally, there would be no appreciable error in the resulting measurement based on ratios of these two light measurements, since the ratio would not be affected.
Prior art circuitry for driving or energizing light sources to be used in this manner were burdened with problems caused by variations of ambient temperature, supply voltage, etc. Tungsten filament lamps combined with light-chopper wheels were employed; different wavelength light was obtained by passing the light through different filters (colored glass). This arrangement was extremely dependent upon voltage and operating life. The wavelength and intensity both shifted with use. Operating life was limited, since circuit parameters varied over long periods of time. Furthermore, in prior art drive circuitry usually multi-current sources were used, one for each diode. Thus, variations between individual sources created additional error. Accordingly, measurements made using prior art equipment had certain limitations and inaccuracies. The present invention provides a solution to these severe prior art problems.